If it's of any concern to someone - we have hit the bug of upgrade
(not downgrade as Oracle support insists). The problem has something
to do with synonym for DBMS_STANDARD. Apparently it's not "normal"
synonym in a way that practically anything depends on it and it's
implemented somehow differently as I've been told. During the upgrade
procedure something goes wrong with this synonym and it results in
exact behavior we were facing.
Workaround - flushing shared pool (I wasn't able to verify it). We
used our workaround - compile DBMS_STANDARD package and package body
and than running utlrp.sql. Apparently, support says that it's not
that easy to compile dbms_standard and what we've done doesn't really
compile it - they claim that it's only possible to compile
dbms_standard in migrate/restrict mode.
Anyway, I don't think that 100% of above is exactly true but I was
given that info. Bug numbers are 4710649 and 4882839 fixed in 11.0.

Thanks to all replied.

2006/5/15, Alex Gorbachev <gorbyx_at_gmail.com>:
> Hi all,>> This is very weird... Third time in the last few days catproc got> invalidated. This is on 10.1.0.4 database on AIX that was upgraded to> 10.2.0.2 and than downgraded back to 10.1.0.4 (almost a month ago).> it's a DW environment.>> Last occurrence's description:> The issue appears in different forms one of which is trying to runDBMS_STATS:
> ORA-04068: existing state of packages has been discarded> ORA-04065: not executed, altered or dropped stored procedure"SYS.DBMS_STANDARD"
> ORA-06508: PL/SQL: could not find program unit being called> ORA-06512: at "SYS.DBMS_STATS", line 12210> ORA-06512: at line 2